1. Field of the Invention
The present invention relates to an electrical wiring structure, a liquid ejection head, a liquid ejection apparatus, and an image forming apparatus, and more particularly to an electrical wiring structure, a liquid ejection head, a liquid ejection apparatus, and an image forming apparatus, which guarantee the reliability of electrical connections even in cases where mechanical vibrations are applied.
2. Description of the Related Art
An inkjet recording apparatus comprises a recording head having a plurality of nozzles and forms a desired image on a recording medium by ejecting ink droplets from respective nozzles, while moving the recording head relatively with respect to the recording medium. In the recording head, a large number of electrical wires are provided in order to transmit drive signals for causing ink droplets to be ejected from the respective nozzles, and one important technological aspect is guaranteeing the reliability of the electrical connections where these electrical wires are connected. In particular, in the case of a method which ejects ink droplets from nozzles by pressurizing the ink inside pressure chambers by using the displacement of piezoelectric elements (a piezoelectric method), since a mechanical vibration is applied to the electrical connections, then it is important to guarantee the reliability of the electrical connections.
As a structure of electrical wires provided in a recording head, for example, there is a structure in which the individual electrodes of piezoelectric elements formed in one surface of a substrate are connected to lead electrodes formed in the other surface of the substrate via through electrodes which pass in the thickness direction of the substrate. The generally known methods of forming the through electrodes includes a method where conductive paste is filled into through holes formed in the thickness direction of the substrate, and a method where conductive material is deposited by plating. However, since a method based on filling conductive paste guarantees electrical conductivity by means of the conductive particles dispersed in the conductive paste mechanically making point contacts with the connected electrode (the individual electrode, lead electrode, or the like), the electrical resistance in the electrical connections is high and there is a possibility that conductivity may not be guaranteed if foreign matter is present on the connection surfaces. In particular, there is a possibility of disconnection in cases where a mechanism vibration is applied to the electrical connection. On the other hand, a plating method is restricted to substrates which can be used in a plating solution (for example, glass substrates cannot be used with strong alkalis), and furthermore, since the resulting through electrodes are hard, then there is a possibility that cracks may occur and disconnections may arise in the through electrodes if mechanical vibrations are applied. In this way, in either of these methods, it is difficult to guarantee reliability in the electrical connection sections in cases where mechanical vibrations are applied.
In this respect, Japanese Patent Application Publication No. 7-240582 and Japanese Patent Application Publication No. 2003-234576 disclose the techniques where electrical connection sections are made from an alloy material, using tin which is a low-melting-point metal (melting point: 232° C.), in order to ensure reliability of the electrical connections and good workability.
Although the electrical connections described in Japanese Patent Application Publication No. 7-240582 and Japanese Patent Application Publication No. 2003-234576 are in a liquid state when heat and pressure are applied during the formation of the electrical connections, they assume a solid state in the operational temperature range which is in the region of room temperature (room temperature to 150° C.). Therefore, if a mechanical vibration is applied to the electrical connections, then the disconnection may occur due to this vibration, and hence there is concern for reliability in the electrical connections.